Local electrical power generating systems are used to provide emergency power for facilities where loss of power can result in catastrophic or fatal consequences. For example hospitals normally have backup power generators in the event of a power failure to power the equipment necessary for patient care and to insure electrical supply to surgeries and the like. Similarly, certain industrial processes cannot be interrupted without substantial financial loss and in some cases exposure to toxic or explosive gases
More recently, power fluctuations resulting in “brown outs” have raised interest in backup electrical generating equipment. In addition, the production of local power can result in lower usage of externally generated power with a resulting reduction in the utility bill from the external power company.
Basically, a local power generating system consists of a suitable motor or other power source that drives a generator for production of electrical power. A concern with any such system is the emission from the power source and the efficiency of the power source that drives the electrical generator of the system.
Various engines are used or have been proposed for use as the power source. Most commonly used is the conventional internal combustion engine (Otto cycle) or the diesel engine. (compression ignition). External combustion engines, such as the steam engine (Rankine cycle), and hot air engines (Sterling cycle) have also been utilized. More recently microturbines based on the gas turbine engine (Brayton cycle) have become reasonably priced and are now widely used. Electrochemical engines (fuel cells) have also been considered for use as the power source for local generating systems.
Although suitable for the purpose, the above mentioned engines are subject to certain deficiencies that can reduce the utility and increase the expense of operation of the local power generating system.
For example, diesel engines are efficient but the emissions from the engine do not meet air quality standards for stationary engines. In most cases particulate traps are required and low sulfur fuel is highly preferred for operation of the engine, raising the maintenance and operational costs of the generating system. Internal combustion engines, although undoubtedly the most widely used engine for local generating systems, are inefficient and are subject to emissions problems.
Both the Rankine cycle engine and the Sterling cycle engine are efficient and can meet emissions standards. However, both types of engines are heavy and may require structural modifications of the installation site. In addition these engines are expensive to manufacture.
Fuel cells also are rated good to excellent in both efficiency and emissions but more work is needed to make them more reliable. In addition, their cost at this time is prohibitive.
Microturbines comprising a compressor and an expander have become popular as power sources for local area electric generation. However, as power sources the are inefficient and expensive although the are generally rated as good with regard to emissions. Gas turbines operate at high speed subjecting the components of the engine to high stresses. The high speeds at which the turbine engines operate require the use of expensive electrical generators additionally raising the overall cost of the system. Also, existing materials of construction limit the operating temperatures that adversely affect the efficiency of the engine.